Stack Concept and LIFO Principle in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA Java

stack.[1](10);

Drag options to blanks, or click blank then click option'

Aremove

Bpop

Cpeek

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push will remove an element instead of adding.

Using peek does not add an element, it only shows the top element.

The push method adds an element to the top of the stack.

2fill in blank

medium

Complete the code to remove the top element from the stack.

DSA Java

int top = stack.[1]();

Drag options to blanks, or click blank then click option'

Apeek

Bpush

Cpop

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using push will add an element instead of removing.

Using peek only shows the top element without removing it.

The pop method removes and returns the top element of the stack.

3fill in blank

hard

Fix the error in the code to check the top element without removing it.

DSA Java

int top = stack.[1]();

Drag options to blanks, or click blank then click option'

Apeek

Bpush

Cpop

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop removes the element, which is not desired here.

Using push or remove are incorrect for this purpose.

The peek method returns the top element without removing it.

4fill in blank

hard

Fill both blanks to create a stack and add an element 5.

DSA Java

Stack<Integer> stack = new [1]<>();
stack.[2](5);

Drag options to blanks, or click blank then click option'

AStack

Bpush

Cpop

DQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Queue instead of Stack for the object.

Using pop instead of push to add an element.

We create a new Stack object and use push to add 5.

5fill in blank

hard

Fill all three blanks to check if the stack is empty and print a message.

DSA Java

if (stack.[1]()) {
    System.out.println("Stack is [2].");
} else {
    System.out.println("Stack is [3].");
}

Drag options to blanks, or click blank then click option'

AisEmpty

Bempty

Cnot empty

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'empty()' method which does not exist in Java Stack.

Swapping the printed messages.

The isEmpty() method checks if the stack has no elements. We print "empty" if true, otherwise "not empty".

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. First In, First Out - the first element added is the first to be removed

B. Last In, First Out - the last element added is the first to be removed

C. Elements are removed randomly

D. Elements are removed based on their value

Solution

1. Step 1: Understand the LIFO principle

   LIFO means the last item you put in is the first one you take out.

2. Step 2: Apply to stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option B

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack like a stack of plates, last on top removed first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking elements are removed randomly
• Assuming removal depends on element value

2. Which of the following is the correct way to push an element '5' onto a stack named stack in Java?

easy

A. stack.insert(5);

B. stack.add(5);

C. stack.put(5);

D. stack.push(5);

Solution

1. Step 1: Recall Java Stack method for adding elements

   The method to add an element to a stack is push().

2. Step 2: Match method with given options

   Only stack.push(5); correctly uses the stack push method.

3. Final Answer:

   stack.push(5); -> Option D

4. Quick Check:

   Use push() to add elements to stack [OK]

Hint: Use push() to add elements to stack in Java [OK]

Common Mistakes:

• Using add() which is for lists, not stacks
• Using insert() or put() which are invalid for Stack
• Confusing stack methods with other collections

3. What will be the output of the following Java code?

Stack<Integer> stack = new Stack<>();
stack.push(10);
stack.push(20);
stack.push(30);
System.out.println(stack.pop());
System.out.println(stack.pop());

medium

A. 30\n20

B. 30\n10

C. 20\n30

D. 10\n20

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 10 (bottom), 20, 30 (top).

2. Step 2: Trace pop operations

   First pop removes 30, second pop removes 20.

3. Final Answer:

   30\n20 -> Option A

4. Quick Check:

   Stack pops last pushed first: 30 then 20 [OK]

Hint: Pop returns last pushed element first [OK]

Common Mistakes:

• Assuming pop removes from bottom
• Mixing order of popped elements
• Confusing push and pop effects

4. Identify the error in the following Java code snippet using a stack:

Stack<String> stack = new Stack<>();
stack.push("apple");
stack.push("banana");
String fruit = stack.pop();
System.out.println(stack.peek());
stack.pop();
stack.pop();

medium

A. Using peek() instead of pop() is incorrect

B. Stack cannot hold String objects

C. Calling pop() on an empty stack causes EmptyStackException

D. push() method is not defined for Stack

Solution

1. Step 1: Trace stack operations

   Push "apple", then "banana". Pop removes "banana". Peek shows "apple". Next pop removes "apple".

2. Step 2: Identify error on last pop

   Last pop tries to remove from empty stack, causing EmptyStackException.

3. Final Answer:

   Calling pop() on an empty stack causes EmptyStackException -> Option C

4. Quick Check:

   Pop on empty stack throws exception [OK]

Hint: Pop more times than pushes causes error [OK]

Common Mistakes:

• Ignoring exception on empty pop
• Thinking peek removes element
• Believing stack can't hold Strings

5. Given a stack with elements pushed in this order: 1, 2, 3, 4, 5. Which sequence of pop operations will produce the output 5 4 3 and leave the stack with elements 1 2 remaining?

hard

A. Pop three times consecutively

B. Pop once, push 6, then pop twice

C. Pop twice, push 7, then pop once

D. Pop once, pop twice, push 8

Solution

1. Step 1: Understand initial stack order

   Stack top is 5, bottom is 1 after pushing 1 to 5.

2. Step 2: Pop three times consecutively

   Pop removes 5, then 4, then 3, leaving 1 and 2 in stack.

3. Final Answer:

   Pop three times consecutively -> Option A

4. Quick Check:

   3 pops remove top 3 elements [OK]

Hint: Pop as many times as output elements needed [OK]

Common Mistakes:

• Adding elements unnecessarily
• Confusing push and pop order
• Expecting output without popping